The present-day method of limiting a critical variable--discharge pressure or suction pressure or pressure ratio--of either a group of compressors (compressor station) or an individual compressor, is carried out by two Proportional-Integral-Derivative (PID) loops: (1) a main-controller loop that acts on the performance of the compressors, such as rotational speed by way of speed controllers, or guide-vane position by way of performance controllers; and (2) a limiting-controller loop of the critical variable that acts on antisurge (recycle) valves by way of antisurge controllers. However, the effect of the main controller (as defined by gains of proportional, integral, and derivative components) is limited by stability conditions for controlling an inertial object like a gas pipeline, and by the rate of load changes (if high) of the gas pumping equipment (including drivers), which can reduce the pumping equipments' longevity.
In its current operating mode, a limiting controller often cannot prevent an emergency shutdown after encountering large, fast disturbances because its set point is very close to an emergency shutdown set point. Thus, the limiting controller initiates action to start opening the recycle valves when the critical variable is close to the emergency shutdown set point. But the stroke time of these valves is usually not less than 2 seconds, and there may be significant volume in the recycle loops; therefore, opening the valves might not prevent emergency shutdown.
Consequently, at the onset of sudden, large changes of mass flow at a compressor's or a compressor station's discharge, the effectiveness of the main and limiting controllers (acting independently) may be insufficient to prevent discharge pressure from reaching the shutdown set point which, in turn, disrupts the process supported by the compressor(s) and decreases the life of the driver(s). Similar problems might occur while limiting the suction pressure in the event of sudden, large changes of mass flow in a compressor's or a compressor station's suction.